Circuit interrupters, such as GFCI and AFCI, are electronic devices connected to power lines. They require having protection against surges known to “travel” in power lines. UL 943 standard for GFCIs and UL 1699 for AFCIs require these devices to pass unwanted tripping tests for combination waveform and surge immunity tests at levels of 2 kV (1 kA), 4 kV (2 kA) and 6 kV (3 kA).
To protect circuit interrupting electronic devices against the power line surge, it is commonly known to use MOV (metal oxide varistors) devices. MOV devices are capable of clamping voltage at a predetermined level and dissipating surge energy up to another predetermined level.
Known circuit interrupting products typically include an MOV positioned across the power lines of the circuit interrupting product, with the MOV providing some surge protection to the circuit interrupting product circuitry by clamping transient voltages to acceptable levels.
An MOV is typically a non-linear resistance that has a very high resistance below its threshold voltage and is typically modeled as an open circuit. At voltages above the threshold voltage, the resistance is nearly zero and the voltage above the threshold is dissipated. The amount of energy that an MOV dissipates is generally related to the size of the device, typically a disc or 9, 10, 14, 20, or 40 mm or the like. A larger MOV typically dissipates more energy, but takes up more space, may be more costly and may require more open space around the device.
The nature of the clamping and the amount of energy that may be dissipated is determined by the size of the disc and voltage rating associated with a disc type MOV. Heretofore, GFCI/AFCI and other circuit interrupting products have typically been limited to handling transient voltages of 6 kV at 3000 A.
When overvoltage conditions occur, protection components such as the MOV in the typical GFCI and AFCI may not survive, if they are selected to only operate under 120V conditions. For example, a MOV in the typical GFCI operating beyond its rating at overvoltage may disintegrate, and thus such conditions may also destroy the rest of the electronics in the GFCI product and possibly cause a fire. Furthermore, an MOV may fail by rupturing, exploding or igniting. Such failure conditions are potentially dangerous.
Due to the requirements of circuit protection products withstanding abnormal overvoltage conditions, 120V devices cannot use MOV rated below 230V-240V. In the event of surge pulses, such devices cannot clamp voltage fast enough so the peak voltage during surge can reach 600V-800V which may cause electronic components of the devices to malfunction or the device may trip which is undesired. For this reason, additional protection is needed to better protect GFCI/AFCI and other circuit interrupting devices and satisfy UL standard requirements.